The field of the invention is medical treatments generally and patient vascular access systems. The present invention relates to embodiments of a method and a system for detecting disconnection of an access needle or catheter while receiving medical treatment.
The maxim of “first, do no harm,” may be a good summary of the Hippocratic oath required of doctors and practiced by medical professionals. Nowhere is this principle required more than in modern medicine. With patients living longer, there are more extended treatments and more frail patients than ever. Such patients are in danger from a number of complications that can arise from continuing therapeutic procedures, and even from diagnostic procedures, that are necessary for their continued care. Treatments involving extra-corporeal blood treatment are clear examples.
The most obvious danger is infection, but the harm caused by infection can be overcome by not re-using even supposedly-sterile devices, by diligent attention by the patient himself or herself, and by the careful attention of care givers attending the patient. Other problems also arise, but, like infections, have been difficult to eradicate. One of the problems arises in blood treatment procedures in which the patient's blood is physically removed for treatment and then returned, all in the same procedure. Removal and return of blood is practiced in hemodialysis, for those persons whose kidneys do not function well. Other procedures, such as apheresis, involve removing blood from a patient or a donor, subjecting the blood to a centrifugal treatment to separate blood platelets or plasma from red blood cells, and then returning the red blood cells to the patient or donor, as described in U.S. Pat. Nos. 5,427,695 and 6,071,421.
The extracorporeal medical treatments described above require that the blood be removed for treatment and then returned. This requires access to the patient's vascular system, from which blood is removed and to which blood is then returned. If a “batch” treatment is used, that is, a quantity of blood is withdrawn, treated and returned, only a single needle is used. Each batch treatment is typically short, and the treatment is attended by a medical professional at a clinic or hospital. Other treatments are continuous, such as the platelet separation discussed above, or dialysis treatment, and may require a duration of several hours or even overnight. There is also a “batch continuous” method in which a single needle is used. There are distinct draw and return phases of a batch continuous process. During the draw phase, blood is processed and additional blood is sent to a holding container to the processed during the return phase. During the return phase, blood is processed from the holding container and returned to the patient/donor through the single needle.
Continuous treatments require two needles, or access points, one for withdrawal of blood and one for return. The withdrawal site is normally an artery although a vein could also be used, and a needle and a pump are used to provide the blood to the therapeutic machine. It is relatively simple to detect a problem with withdrawal, for instance, if the withdrawal needle is dislodged. Bubbles form in the withdrawal line and conventional air sensors detect the bubbles. Detecting a problem in the return of the blood to the patient is more difficult. The return line typically includes a needle with venous access. If the return line is dislodged, the blood is not returned to the patient, but may continue to be pumped and may accumulate near the patient, but not returned to the patient's vascular system. Depending on the pumping rate of the blood and the time for treatment, this could have life-threatening effects on the patient within a very short period time.
Accordingly, a number of apparatuses have been devised for detecting needle dislodgement, especially venous dislodgement. An example is U.S. Pat. Appl. Publ. 2006/0130591. In a device according to this application, a venous needle is equipped with a photosensor and is covered with an opaque patch. This device would not send a signal or an alarm if the needle begins leaking or is only slightly dislodged. The photosensor could also fail to detect light because the needle has not been dislodged sufficiently to expose the photosensor to light.
Another example is U.S. Pat. No. 7,052,480, in which an induction coil is attached to a venous blood line and a second coil is attached to the arterial blood line. An electric current is generated and injected into the blood circuit, passing from one coil through the blood to the second coil. Signal processing circuitry is able to detect any variation in the resulting current in the second coil, and can interpret any significant change in the current, impedance, voltage, resistance, and so forth, as an indication of a leak or of dislodgement. This technique is complicated and difficult to set up for consistent, reliable operation, in consideration of natural variances over long periods of time, and also considering differences between patients.
Another example is provided in U.S. Pat. No. 7,060,047, in which an electric signal is also injected into the patient and is used to form a capacitor. A ring electrode is placed around the tubing of the venous branch, between the drip chamber of a hemodialysis machine and the access needle returning blood to the patient. The blood of the patient forms the other plate of the capacitor. An electronic circuit is used to monitor the voltage across the plates of the capacitor. When the venous access needle is dislodged, a large change in the voltage results, and is detected by the electronic circuit. This technique has the disadvantage that a voltage change may not result until the blood has been exhausted and no longer fills the tubing in the return line.
Numerous other techniques have been devised, many of them depending on a flow of blood causing conductivity between two electrodes or two wires. One problem with these methods is that it is not easy to distinguish between small amounts of blood and the natural perspiration of a person, which can cause false alarms. What is needed is a better way of quickly detecting dislodgement of a venous or other needle or catheter from a patient, so that inadvertent loss of blood and harm to the patient is avoided.